C6H12O6 + 6 O2 ----> 6 CO2 + 6 H2O DHo=-686 kcal/mole
The free-energy change of the above reaction is -686 kcal/mol
glucose under standard conditions. Hydrolysis reaction of ATP to ADP
ATP + H2O ----> ADP + Pi DHo=-7.3 kcal/mole
under standard conditions has a free-energy change of -7.3
kcal/mol. How may moles of ATP are generated per mole of glucose
metabolized via glycolysis/fermentation (without oxygen)? In the
presence of oxygen, cells can continue with oxidative
phosphorylation to generate more ATP. With oxygen to convert
glucose to carbon dioxide, how many moles of ATP are generated
for each mole of glucose metabolized? Based on these numbers,
calculate and compare the energy efficiencies of the
glycolysis/fermentation pathway and the oxidative phosphorylation
pathway. Here we define energy efficiency to be the ratio of the
amount of energy recovered in ATP over that potentially
achievable by burning glucose under standard conditions. Do you
expect the actual energy efficiency to be higher or lower in
living organisms? Why?
Solution:
Energy recovered from glucose metabolism in the absence/presence of oxygen.
At intracellular conditions, the free energy is about -13
kcal/mole of phosphate bond. Re-working with this number gives
higher energy efficiencies.
Presence of Oxygen ATP Energy Efficiency
-----------------------------------------------------------------------
Without oxygen (Glycolysis) 2 (7.3* 2)/686= 2%
With oxygen (Glycolysis+TCA+Respiration) 36 (7.3*36)/686=38%
Note that you cannot jump to TCA without going through glycolysis first
to generate pyruvate.
Likewise, you need glycolysis and TCA to generate NADH and FADH for
oxidative phosphorylation.
The baseline for comparision is the amount of energy recovered from each
mole of glucose.
The actual efficiency in living organisms is higher because reactions
proceed in non-standard conditions (e.g., less than 1M, other temprature.)
Presence of Oxygen ATP Energy Efficiency
-----------------------------------------------------------------------
Without oxygen (Glycolysis) 2 (13* 2)/686= 4%
With oxygen (Glycolysis+TCA+Respiration) 36 (13*36)/686=68%
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